At present, an OTDR (Optical Time Domain Reflectometer, optical time domain reflectometer) is test equipment widely applied in optical fiber networks. A basic principle of the OTDR is sending probe light to an optical fiber, and receiving a reflected/scattered optical signal on an optical fiber link. Through analyzing and processing a transmitted signal and the received reflected/scattered signal, a real state of the optical fiber link including line attenuation and a type and position of an event point (bending, connector or fusion splice point) may be reflected. The OTDR is capable of implementing recognition and location of various events on the optical fiber link in a single-end test manner; therefore, the OTDR plays an important role in test, fault location and troubleshooting of an ODN (Optical Distribution Network, optical distribution network) in a PON (Passive Optical Network, passive optical network) system. In order to decrease cost, simplify a network structure and reduce the influence of a test signal on data communication, it is brought forward in the industry to integrate the OTDR to an optical module of an OLT (Optical Line Terminal, optical line terminal), to generally reuse a data transmitter in the optical module of the OLT as a transmitter of the OTDR, and to insert the test signal into a data signal or bear the test signal after modulation on the data signal, that is, using a wavelength for data transmission as a test wavelength to bear the test signal. In one implementation solution, reusing a LD (Laser Diode, laser diode) in the optical module as an OTDR LD, and reusing a MPD (Monitor Photo Diode, monitor photo diode) in the optical module as an OTDR PD is adopted as a hardware implementation manner; while for software control, the following manner is adopted: the OLT first sends an OMCI (ONT Management Control Interface, ONT management control interface) message to notify an ONU (Optical Network Unit, optical network unit) or an ONT (Optical Network Terminal, optical network terminal) to perform OTDR test setting; the ONU needs to perform clock locking, and sets a test start time and a test end time, and then the OLT inserts test data to downlink data for test through a specific identifier; the ONU does not perform clock extraction during the test; and the ONU quickly performs clock recovery when the test ends. However, the ONU is easily disconnected during the test, thereby causing service interruption.